Question

1. Provide an example and describe the situation where it is difficult to determine the difference between incidence and prevalence.
2. Discuss the primary differences between attack rate, case fatality rate, and incidence? Why are these often misunderstood?

Answer 1

Incidence is often confused with prevalence. The easy way to remember the difference is that prevalence is the proportion of cases in the population at a given time rather than rate of occurrence of new cases. Thus, incidence conveys information about the risk of contracting the disease, whereas prevalence indicates how widespread the disease is.

Prevalence gives a figure for a factor (disease, injury, health status etc) at a single point in time (point prevalence) or time period (period prevalence). Period prevalence provides the better measure of the factor since it includes all cases between two dates, whereas point prevalence only counts cases on a particular date. It is a measure of disease that allows us to determine a person's likelihood of having a disease. It is most meaningfully reported as the number of cases as a fraction of the total population at risk and can be further categorised according to different subsets of the population.
An example of prevalence: A recent Scottish study showed that the prevalence of obesity in a group of children aged from 3 to 4 years was 12.8% at the time.

Incidence is the number of instances of a factor (disease, injury, health status etc) during a given period (day, month, year, decade) in a specified population (age group, community, country etc). Incidence can tell us how many cases of a particular factor have been suffered by a specified population in a given period of time. It is a measure of disease that allows us to determine a person's probability of being diagnosed with a disease during a given period of time or it might tell us how patterns of a condition within a population change over time.
An example of incidence: Auckland in New Zealand, often has epidemics of meningococcal disease, with annual incidences of up to 16.9/ 100,000 people.

Attack rate is the number of people at risk in whom a certain illness develops/total number of people at risk.
Case fatality rate is the number of deaths caused by a disease among those who have the disease.

Incidence rate is the number of new cases of a disease that occur during a specified period of time in a population at risk for developing the disease.